In typical synthetic aperture radar systems a plurality of antenna elements are provided, through which both high power signals are emitted and corresponding low power echoes are received. So-called transmit receive (T/R) modules have the function of switching between incoming and outgoing signals and to provide input and output phase adjustment as well as input amplification.
It is widely known to use ferrite circulators in order to separate incoming and outgoing signals. However, circulators are only effective up to certain magnitudes of power.
Another drawback with ferrite circulators is their bulk and weight.
It is known to use passive limiters for protecting the input section of a radar system.
Prior art document EP-A-0391326 discloses a receiving transmission line comprising an antenna input post, over which signals are transmitted to a receiver. The transmission line comprises two PIN diodes. The PIN diodes are adapted to be selectively biased for reflecting signals entering the transmission line and thereby protecting the input stage and adapted for being selectively unbiased for rendering the parallel-coupled PIN diodes yielding a high impedance and thereby opening the transmission line for signals to be received. Protection is accomplished also when the equipment is switched off.
Prior art document U.S. Pat. No. 4,232,278 shows a radar receiver protector circuit having a transmission line input stage comprising first, second and third parallel coupled PIN diodes. The circuit discloses sensitivity time control, functioning as a passive limiter during transmit and controlled attenuator during receive.
Prior art document U.S. Pat. No. 4,789,846 shows a microwave semiconductor switch, for instance for switching an antenna between transmission and reception modes wherein first and second field effect transistors and first, second and third input/output microstrip lines are integrally formed on a semiconductor substrate. The first field effect transistor is connected in series between the second input/output line and a junction point of the first second and third input/output lines at a position spaced approximately a quarter of the wavelength from the junction point. The second field effect transistor is connected at a second position spaced approximately a quarter of the wavelength from the junction point between the second position and the ground. The transmission paths for microwaves are switched by varying a bias voltage applied to the gate electrodes of the field effect transistors. The switch offers no protection against incoming high-energy signals.